Pickup tube



June 20, 1961 R. c. PALMER 2,989,658

PICKUP TUBE u Filed Aug. 22, 1958 loAW I r Is b d Fig. mm

2O TEMPERATURE COMPENSATING CIRCUIT INVENTOR. RICHARD c. PALMER BY L A TTORNEYS United States Patent are Filed Aug. 22, 1958, Ser. No. 756,687 5 Claims. (Cl. 313-65) This invention relates to pickup tubes, and more particularly to a pickup. tube which responds to infra red radiation, and/ or other radiations which may be converted to heat.

Many pickup tubes, photocells, and similar devices are based upon the principle that when light impinges upon a target formed from a material known as a photoconductor, the conductivity thereof changes. When a voltage is impressed across this material, the changes in conductivity produce changes in the amount'of current which is permitted to flow. In this way, the fluctuations in the current correspond to the impinging light.

-It is frequently desired to map, view, detect or follow objects in the absence of visible light. This result is possible because warm but non-luminous objects emit infra red radiation. By utilizing a pickup device which contains target comprising a material which changes its conductivity when struck by infra red radiations, these changes can be caused to produce corresponding current fluctuations.

In general, both photoconductive and infra red responsive target materials have peaked response curves which means that they respond particularly well to certain wavelengths, but are relatively insensitive to other frequencies. Pickup devices having this type of response are at a disadvantage when the radiations to be detected are in the insensitive range of the materials response curve.

Other target materials, such as oxides of manganese (as used in thermistors) tend to be equally responsive to all the frequencies in a wide spectrum. In order to utilize these latter materials as targets they or their containers, are blackened or otherwise caused to absorb all of the impinging radiations. These radiations, upon absorption, warm the target material which therefore changes its conductivity. In the same manner as previously described, the changes in conductivity cause changes in an electrical current, whose fluctuations then correspond to the impinging radiations. In this Way, thermistor type materials may be used as targets for pickup devices designed to operate in complete darkness.

Unfortunately, many of the materials which are most sensitive to infra red radiations and to heat and therefore most desirable in the above type of pickup devices, have an inherently low resistance (or stated in another way, a high conductivity) in the absence of irradiation. The application of a voltage to the unilluminated target causes the flow of a dark current, which has a relatively high amplitude. While the materials resistance is further reduced when irradiated, the ever present high amplitude dark current tends to mask the signal current, and to produce an undesirably low signal-to-noise ratio.

It is therefore the principal object of my invention to provide an improved pickup device.

It is another object of my invention to provide a pickup device which is more sensitive to heat and/or infra red radiation than prior art devices.

The attainment of these objects and others will be realized from the following specification, taken in conjunction with the drawings, in which,

FIG. 1 is a diagram illustrating the relationship between various currents of a pickup tube; and

FIG. 2 is a diagrammatic representation of a pickup tube utilizing my invention.

Basically my invention contemplates a novel pickup tube wherein one electron gun supplies the dark current, while a second electron gun supplies the signal current. In this way, the masking effect of the dark current is eliminated, and the signal-to-noise ratio is improved.

The problem to be overcome may be understood from a study of FIG. 1, which shows the relationship between currents in a prior art pickup tube. In FIG. 1 there is illustrated a target 10 whose conductivity varies according to external excitation (which is to be construed as including infra red radiations impinging upon it), or other radiations which change its temperature. A beam of electrons 1,, is directed toward target 10 from the cathode of an electron gun. In the absence of external excitation, the dark current component I is supplied by beam current I while the remainder I, is returned to the output circuits. When excitation strikes target 10, its conductivity changes, and an additional signal current component I is taken from the beam current, still further reducing the return current 1,. It is this additional reduction which is of interest. The signal current component I is theoretically the only variable, and therefore the changes in I represent the excitation impinging upon the target. Since the dark current component I is large (for the reasons previously given), the beam current l which supplies these several components, must be extremely large. The variations of the signal component I are therefore only a small percentage of the beam current I producing a low signal-to-noise ratio. In addition, any slight variations in the dark current may be erroneously interpreted as a signal current.

FIG. 2 illustrates a pickup tube 12 utilizing my invention. Tube 12 contains a main gun 14, and an electron multiplier 16 for producing an output signal from the return current 1,. A flood gun 18 capable of producing a beam of electrons I is positioned so that its electrons are deposited uniformly on every portion of target 10. Flood guns are well known in the art of storage tubes, and will therefore not be discussed indetail.

My invention operates as follows. The intensity of the electron beam from flood gun 18 is adjusted so that it is exactly equal to the dark current. In the absence of impinging excitations, there is no signal current, and the current If from the flood gun supplies the dark current I The beam current l from main gun 14 is therefore equal to the return current 1,. When excitations impinge on target 10 they produce a signal current I which is subtracted from the beam current I the reduced return beam 1,, striking electron multiplier structure 16 in the well known manner to produce an output signal. In this way the electron beam from main gun 14 is divorced from the dark current, and all variations of the return current I are the result of changes in the resistance of the target. Since in my invention the beam current l supplies the signal current but does not supply the dark current, l can be just a little larger than 1,. Therefore, the modulation, or variation of I relative to the beam current 1 is high thus producing a desirably high signalto-noise ratio.

Flood guns such as 18 usually provide a wide divergent beam of electrons which cover the entire target. Alternatively, it may produce a thin electron beam which is deflected to sweep across target 10. This alternative has the advantage that if target Iii is not homogeneous, the electron beam from flood gun 18 may be controlled to deposit more or less electrons on specific areas of target 10, and thus compensate for the lack of homogeneity.

Most target materials change their conductivity with changes in ambient temperature. These changes, of course, affect the magnitude of the dark current. In those cases where it is undesirable that the dark current vary Patented June 20, 1961 with changes in the ambient temperature, a compensating circuit 20 may be used to vary the electron beam produced by the flood gun. Circuit 20 may, for example, comprise a temperature sensitive device such as a thermocouple whose output is applied to the control grid of the flood gun.

What is claimed is:

1. An infrared pickup tube comprising: an evacuated envelope having a faceplate, a radiation sensitive conductive target on the inner surface of said faceplate, said target being adapted to produce discrete areas of greater or less resistance in accordance with the intensity of an infrared image focused thereon, said target having a sufliciently low overall resistance to produce a dark current in the absence of an image, an electron gun positioned in said envelope at a point remote from said target to direct an electron beam at said target, means whereby said gun supplies the signal current of said target, said means comprising a source of potential connected between said gun and said target; a collector electrode to receive beam electrons rejected by said target, said collector electrode being adjacent said electron gun, an electron multiplier cooperating with said second electrode, a second electron gun positioned to direct a second electron beam at said target, said second beam supplying only said dark current; and a source of potential connected between said target and said second gun.

2. The apparatus of claim 1 wherein said second beam is divergent and deposits uniformly on every portion of said target. a

3. The apparatus of claim '1 wherein said second beam is focussed and scans across said target.

4. The apparatus of claim 3 wherein the intensity of said second beam is controlled to selectively deposit electrons on specific areas of said target.

5. The apparatus of claim 1 including means to compensate for changes in said dark current due to changes in the ambient temperature at said target, said means comprising a temperature compensating circuit associated with said second electron gun and acting to control the electron beam therefrom whereby changes in said dark current due to variations in said ambient temperature are supplied by said second electron gun.

References Cited in the file of this patent Teal Jan. 18, 1944 Jacobs Apr. 19, 1955 Vance et al. Feb. 21, 1939 

